FTIR studies on phase transitions of N-octadecanoyl-L-alanine and zinc octadecanoyl-L-alaninate LB films

The phase transitions of N-octadecanoyl-L-alanine and zinc octadecanoyl-L-alaninate LB films have been investigated through variable temperature FTIR spectroscopy. Hydrogen-bonding interactions between the enantiomeric head-groups strengthens the interactions between the alkyl chains so that the transition temperature of the N-octadecanyl-L-alanine LB film (T-c = 122 degrees C) is nearly twice as high as that of the same-chain-length stearic acid LB film (T-c = 65 degrees C). The intermolecular hydrogen-bonding interactions in the doe octadecanoyl-L-alaninate LB film are greatly intensified in comparison to that in the N-octadecanoyl-L-alanine LB film: the relatively weak hydrogen-bonding interaction rn the latter is easily disrupted around its transition temperature (T-c - 122 degrees C) together with an abrupt increase of nu(a)(CH) stretching frequency, while the intensified intermolecular hydrogen-bonding Interaction in the former weakens step by step with temperature along with a monotonic increase of nu(a),(CH2) stretching frequency the zinc octadecanoyl-L-alaninate LB film thus exhibits a different phase behavior from the pure enantiomer LB film. Therefore, the strength of the intermolecular hydrogen bond is closely related to the interchain interaction and the film phase behavior.